This invention relates to a monostable multivibrator circuit. More particularly, it relates to a circuit arrangement for applying a trigger signal to a monostable multivibrator circuit of the non-saturated type in which an amplifier transistor operates in a non-saturated state.
A monostable multivibrator circuit usually has a time constant circuit which uses a capacitor, an amplifier circuit which is connected to the output end of the time constant circuit, a positive feedback circuit which is connected between the output end of the amplifier circuit and the input end of the time constant circuit, and a trigger terminal which is disposed in a predetermined portion of the circuit loop.
The monostable multivibrator circuit has its characteristics limited by various causes.
For example, since an electrostatic capacity existing between the trigger terminal and the earth point of the circuit limits the changing rate of the trigger signal, the changing rate of an input signal of the time constant circuit becomes low. As a result, the timing of the rise or fall of an output signal responsive to the trigger signal lags, and the rate of the rise or fall lowers.
Since the trigger terminal is disposed at a part of the circuit loop, undesirable electrical interference takes place between the trigger terminal and any circuit within the loop through the medium of the loop. On account of the interference with the trigger terminal due to the circuit loop, the operation of a trigger signal supplying circuit is adversely affected, or the trigger level varies in case where an operating period differs in repeated operations. Conversely, on account of the interference with the circuits within the loop due to the trigger terminal, the waveform of the output signal is distorted or the pulse width fluctuates, resulting in influences on the circuit operation in some cases.
The output pulse of the monostable multivibrator circuit constants and characteristics thereof. In a case where the trigger period differs, the output pulse comes to have a duty ratio corresponding to the particular period. It has therefore been considered to apply the monostable multivibrator circuit to, e.g., the so-called pulse count detector circuit in which an FM (frequency-modulated) signal is converted into a pulse signal of a duty ratio corresponding to the frequency, and the pulse signal is smoothed thereby to obtain a demodulated signal.
The monostable multivibrator circuit for use in such a pulse count detector circuit needs to operate at high speed and to deliver pulse signals of fixed amplitude and pulse width having a sharp waveform in order that a duty ratio exactly corresponding to the frequency of the FM signal may be given and that the signal level after the smoothing may exactly correspond to the duty ratio.
Heretofore, there has not been a monostable multivibrator which meets these requirements satisfactorily.